1) Field of the Invention
The present invention relates to an optical amplifier and a method of controlling the optical amplifier. Particularly, the present invention relates to an optical amplifier and a method of controlling the same, which are suited to be used when signal light is amplified in a wavelength division multiplexing (WDM) transmission system.
2) Description of the Related Art
According to the expansion of multi-media networks, the enhancement for communication traffic is a prime task. Therefore, it is much important to utilize WDM transmission systems including optical amplifiers for amplifying and relaying arranged in multiple stage to reduce the cost of the communication systems in the society affected the multi-media networks.
Transmission loss estimated in a WDM transmission system is as very wide as 0 to about 30 dB. When optical amplifiers which compensate transmission losses in a wide range estimated as above are applied to a WDM transmission system, it is general to place many types of optical amplifiers on a menu according to each compensation amount for a transmission loss required at a position where each optical amplifier is applied. However, when many types of optical amplifiers are placed on a menu, there arises a problem of an increase in cost because many types of optical amplifiers have to be in stock and, and a problem of an increase in labor cost and time required to select a type of the optical amplifier. Hence, it is desired to be able to compensate the transmission losses with a small number of types of optical amplifiers.
To meet such demand, the optical amplifier is required to realize three kinds of performance expressed in terms of operation conditions, that is, a constant output power, a flat gain-versus-wavelength characteristic, and a low noise figure (NF), with respect to a wide input dynamic range.
For example, as one of the techniques of WDM transmission systems, including an optical amplifier having a variable optical attenuator (VOA) which is disposed between the front-stage amplifying unit and the rear-stage amplifying unit. Although a person who is not skilled in the art may believe that VOA is unmatched in the amplifiers, such an optical amplifier having VOA is disclosed in Patent Document 1 as below. This configuration is a standard of optical amplifiers in WDM transmission systems. Incidentally, rare-earth-doped optical fibers such as EDFs (Erbium Doped Fibers) are used as the amplifying units in the front and rare stages.
When the optical amplifier is so designed as to provide a predetermined output level Pi, the configuration in which a variable optical attenuator is disposed between the front-stage and the rear-stage optical amplifying units as described in Patent Document 1 can provide the predetermined output signal light level with a wider dynamic range of the input light and an excellent NF by performing the gain constant control, as compared with configurations where (1) the optical amplifier is formed with one optical amplifying unit, (2) a VOA is placed in the rear stage of the optical amplifying unit, and (3) the optical amplifying unit is placed in the rear stage of a VOA.
(1) Namely, when the optical amplifier is configured as a single optical amplifying unit 100 comprising an amplification medium 101 made of an EDF and a pumping source 102 as shown in FIG. 15 (a), it is necessary to decrease the gain in order to keep the output level when the input level changes from #1 to #2 as shown in FIG. 15(b). This is accomplished by changing the pump power. However, this change in gain causes a change in gain-versus-wavelength characteristic, which leads to a change in characteristic of a channel of the WDM signal light.
(2) When an optical amplifier is configured with an optical amplifying unit 100A which has an EDF and a pumping source and is controlled in the automatic gain control (AGC), and a VOA 103 disposed in the rear stage of the optical amplifying unit 100A as shown in FIG. 16(a), a constant gain and a constant gain-versus-wavelength characteristic can be both obtained irrespective of the input level, as shown in FIG. 16(b). However, when the input level is relatively high, it is necessary to raise relatively the pumping light power for the purpose of the automatic gain control and to attenuate the signal light level obtained through the amplification by the VOA 103, which impedes efficient use of the pump power.
(3) When an optical amplifier is configured with a VOA 103 and an optical amplifying unit 100B similar to that (refer to a reference character 100A) shown in FIG. 16(a) disposed in the rear stage of the VOA 103 as shown in FIG. 17(a), the input signal light in a stage before amplification is attenuated, as shown in FIG. 17(b). Therefore, even if the input signal light is amplified by the optical amplifying unit 100B in the rear stage, the NF is degraded.
To the contrary, in a configuration where two optical amplifying units 100A and 100B both controlled in the automatic gain control are arranged tandemly, and a VOA 103 (for the output level constant control) is placed between the optical amplifying units 100A and 100B, it is possible to provide a constant gain-versus-wavelength characteristic by performing the automatic gain control while widening the dynamic range of the input signal level. Further, it is possible to keep an excellent NF while using efficiently the pump power.
In Patent Document 1, the automatic gain control is performed by using the gain characteristics that have gain slopes (inclinations of gain-versus-wavelength characteristic) in directions opposite to each other in the optical amplifying unit in the front stage and the optical amplifying unit in the rear stage, thereby flattening the gain-versus-wavelength characteristic of the signal light amplified by the amplifying unit in the front stage and the amplifying unit in the rear stage.
In an optical amplifier 110 shown in FIG. 19, an automatic gain controller (AGC control unit) 114 performs a control to keep a level ratio of a level of input signal light (Is) inputted to an optical amplifying unit 111 in the front stage to a level of output signal light (Os) outputted from an optical amplifying unit 112 in the rear stage, that is, a gain of the optical amplifier 110. While an attenuation amount controller 115 changes the attenuation amount of a VOA 113 according to an amount of variation in the signal light input level to maintain a predetermined signal light output level while keeping flatness of the output wavelength characteristic. Owing to the control by the automatic gain controller 114 shown in FIG. 19, it is possible to improve the control responsibility with the control circuit for the automatic gain control being shared, as compared with the above-mentioned case where the optical amplifying unit 100A in the front stage and the optical amplifying unit 100B in the rear stage are separately subjected to the automatic gain control as shown in FIG. 18(a).
As above, the known techniques are on the assumption that so long as the optical amplifying units are automatic-gain-controlled, the gain-versus-wavelength characteristic can be kept constant irrespective of arrangement of wavelengths of the input signal light, as shown in FIGS. 18(a) and 19. In other words, when rare-earth-doped optical fiber is used as the amplification medium, the known optical amplifiers flatten the gain on the axis of wavelengths without consideration on an effect of SHB (Spectral Hole Burning) which is a phenomenon that deviation occurs in the gain-versus-wavelength characteristic due to arrangement of wavelengths of the input signal light. Details of the physical phenomenon of SHB are described in Non-Patent Documents 1 through 7, etc., for example.
In more details, in the optical amplifier shown in FIG. 18(a) or 19, a GEQ (Gain Equalizer) is placed in the rear stage of the second-stage optical amplifying unit 100B or 112 to give a loss wavelength characteristic corresponding to the gain-versus-wavelength characteristic controlled constant in the optical amplifier, whereby finally realizing the flatness on the axis of wavelengths of the signal light level.
For example, when WDM signal light having 40 channels (40 wavelengths) and four kinds of WDM signal light at six wavelengths having different wavelength arrangements A, B, C and D are inputted to an EDF, which is one type of rare-earth-doped optical fibers, it can be assumed that they have a uniform gain-versus-wavelength characteristic as shown in FIG. 20(a) if SHB is not taken into consideration. Incidentally, in FIGS. 20(a) through 20(c), the characteristic obtained when the WDM signal light at 40 wavelengths is inputted is shown by “⋄,” and the characteristics obtained when the four kinds of WDM signal light at six wavelengths are inputted are shown by “□,” “Δ,” “X” and “*,” respectively.
When a GEQ having a wavelength characteristic that cancels the uniform gain wavelength characteristic shown in FIG. 20(a) is inserted behind an EDF which forms the optical amplifying unit 100B or 112 [refer to the loss wavelength characteristic of GEQ shown in FIG. 20(b)], the gain-versus-wavelength characteristic of the output from the GEQ is flattened as a result as shown in FIG. 20(c), irrespective of the wavelength arrangement. Incidentally, techniques that adjust an optical level amplified by an EDFA (EDF Amplifier) as does the above GEQ are described in Patent Documents 2 through 4 below, for example.    [Patent Document 1] Specification of Japanese Patent No. 3551418    [Patent Document 2] Japanese Patent Application Laid-Open No. H10-150414    [Patent Document 3] Japanese Patent Application Laid-Open No. H7-28105    [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-252923    [Non-Patent Document 1] Takuya Aizawa et al. “Effect of Spectral-Hole Burning on Multi-Channel EDFA Gain Profile,” In: Proceedings of Conference on Optical Communication 1999 (OFC '99) WG1, P102-104    [Non-Patent Document 2] M. Nishihara, et al., “Characterization and new numerical model of spectral hole burning in broadband erbium-doped fiber amplifier,”, OAA2003, TuD3, 2003.    [Non-Patent Document 3] M. Nishihara, et al., “Impact of spectral hole burning in Multi-channel amplification of EDFA,” OFC2004, FB1, 2004.    [Non-Patent Document 4] Ono, Tabe, “Evaluation of the Quenching Effect on Gain Characteristics in Aluminum-silicate Erbium Doped Fiber by Numerical Simulation,” Rare Earths '04, F0-05, 2004.11.7    [Non-Patent Document 5] Ono, Tabe, Nishihara, Ishikawa, “Gain Spectral Hole Formation Behavior of EDF at low temperature,” 45th Glass and Photonic Materials Discussion, H-3, 2004.11.25    [Non-Patent Document 6] Ono, “Optical Properties and Gain Characteristics of Erbium-Doped Fiber Amplifier,” Public Hearing on Theses for degrees, Kyoto University, 2005    [Non-Patent Document 7] Ono, Tabe, Nishihara, Ishikawa, “Effect of erbium ion concentration on gain spectral hole burning in silica-based erbium-doped fiber,” OFC/NFOEC 2005, OThL1, 2005
However, the above known techniques seek flatness of gain on the axis of wavelengths without a consideration on an effect of SHB (Spectral Hole Burning), which is a phenomenon that generates deviation in gain-versus-wavelength characteristic according to wavelength arrangement of inputted signal light. For this reason, the known techniques have an disadvantage that the gain deviation due to a change in amount of generation of SHB due to wavelength arrangement of inputted signal light cannot be solved.
Now, explanation is made of gain deviation occurring when not WDM signal light at 40 wavelengths but four kinds of WDM signal light at six wavelengths A through D in different wavelength arrangements are inputted, for example, with reference to FIGS. 21(a) through 21(c). Incidentally, in FIGS. 21(a) through 21(c), a characteristic obtained when WDM signal light at 40 wavelengths is inputted is shown by “⋄,” and characteristics obtained when four kinds of WDM signal light at six wavelengths are inputted are shown by “□,” “Δ,” “X” and “*,” respectively.
In the case of the WON signal light at six wavelengths, the gain-versus-wavelength characteristic of the EDF is fluctuated by only a small amount according to the wavelength arrangement due to a change in amount of generation of SHB as compared with the WDM signal light at 40 wavelengths, as shown in FIG. 21(a). Here, it is assumed that the gain-versus-wavelength characteristics are identical irrespective of the wavelength arrangement. Under this assumption, a GEQ is inserted to the rear stage of the EDF [refer to FIG. 21(b)], which GEQ has a wavelength characteristic that cancels the gain-versus-wavelength characteristic obtained when the WDM signal light at 40 wavelengths similar to that shown in FIG. 20(b). In the result, the gain-versus-wavelength characteristic obtained when signal light at 40 wavelengths is inputted is flat, but gain deviation occurs in other wavelength arrangements such as the four kinds of WDM signal lights at six wavelengths A through D, as a result, as shown in FIG. 21(c), for example.
The permissible degree of such gain deviation in communications becomes more severe because of a recent increase in transmission distance and an increase in number of spans. Accordingly, there is a demand to introduce a function of flattening the gain deviation due to SHB into optical amplifiers applied to wavelength-division multiplex optical transmission systems.